Plasma catecholamine response to fetal hypoxemia is not potentiated by naloxone

The interaction between the plasma catecholamine response to hypoxemia and endogenous opiate receptor blockade was investigated in 9 fetal lambs. The animals were randomly assigned to receive either normal saline (control group 1) or two doses of naloxone (group 2 = 0.5 mg/kg; group 3 = 1.0 mg/kg) by bolus injection followed by continuous infusion. Arterial PO2 decreased from 21 to 12 Torr and remained less than 14 Torr for 1 h. Arterial pH declined from 7.40 to a nadir of 7.32 at 1 h. Mean plasma catecholamine concentrations rose promptly from a baseline of 524-546 pg/ml for norepinephrine (NE) and 156-235 pg/ml for epinephrine (E) to reach a peak at 15 min (NE = 2,476-3,276, p less than 0.01; E = 1,856-4,065 pg/ml, p less than 0.01) and remained elevated thereafter. Neither dose of naloxone significantly altered plasma NE or E concentrations. Therefore, we conclude that endogenous opiates do not modulate the sympathoadrenal response to moderately long periods of hypoxemia unaccompanied by acidemia.